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The present invention relates to geometric configurations for heat and mass transfer involving sorption of a sorbate vapor into (absorption) or out of (desorption) a volatile liquid sorbent accompanied by heat exchange with a heat transfer fluid. Diabatic sorption processes are useful in many industrial processes, and especially in absorption refrigeration cycles and absorption power cycles.
Sorption inherently involves mass transfer between vapor and liquid phases. Thus, latent heat is either released (absorption) or acquired (desorption). In adiabatic sorption, that change in latent heat changes the sorbent temperature so much that the extent of sorption is very limited. Therefore, in processes wherein large changes of concentration, i.e., sorbent loading, are desired, heat exchange with an external medium is provided. This is called diabatic sorption.
The heat and mass transfer resistances encountered in sorption processes include the following: heat transfer through the liquid; heat transfer between liquid and vapor; and heat transfer through the vapor; plus mass transfer through the liquid, mass transfer between liquid and vapor, and mass transfer through the vapor. The mass transfer resistance through the vapor is encountered in sorption processes involving a volatile sorbent, i.e., those in which the vapor phase includes appreciable quantities of both sorbate and sorbent molecules. Sorption between ammonia as sorbate and water as sorbent are an example of thisxe2x80x94both phases include appreciable quantities of each species, and the xe2x80x9crelative volatilityxe2x80x9d is the ratio of concentrations of the two phases. The vapor phase mass transfer resistance is normally small with a non-volatile sorbent unless non-condensables are present.
The resistance to heat and mass transfer is frequently found to be much greater in sorption processes which have volatile sorbents, owing to the above vapor phase mass transfer resistance. For example, consider the condensation of pure H2O and of pure NH3 on cooled straight tubes. When an NH3xe2x80x94H2O vapor mixture is supplied to the same tubes, the overall coefficient of partial condensation or absorption can be more than an order of magnitude lower than the pure vapor condensation coefficient. The effect is similar to what occurs when there is severe blanketing with non-condensables. This major increase in heat and mass transfer resistance has caused the sorbers for volatile sorbents to be large and costly, which limits their application.
The prior art discloses many attempts and approaches to overcoming this problem, with only limited success. U.S. Pat. Nos. 5,339,654; 5,572,885; and 5,713,216 disclose shell and concentric coil arrangements adapted for diabatic vapor-liquid contact, which utilize unusual tube geometries. U.S. Pat. No. 6,314,752 discloses a partially flooded counter-current falling film geometry from folded sheet metal, similar to a known industrial configuration. U.S. Pat. No. 5,766,519 and 5,660,049 disclose diabatic sorber geometries based on channels formed by folded sheet metal which incorporate liquid recirculation. U.S. Pat. No. 5,490,393 discloses a diabatic (GAX) absorber comprised of three concentric coils of tubing in a shell, all with the same coiling direction. Other prior art disclosures of concentric coils in a shell used as a diabatic sorber in an absorption cycle include U.S. Pat. Nos. 3,254,507; 3,390,544; 3,423,951; and 4,106,309. U.S. Pat. No. 4,193,268 discloses a concentric coil evaporator. U.S. Pat. No. 2,826,049 discloses a co-current downflow NH3xe2x80x94H2O absorber with counter-current heat exchange in a shell-and-tube geometry. An absorption power cycle with a shell-and-coil absorber is disclosed in U.S. Pat. No. 4,307,572. The absorber has crosscurrent mass exchange and co-current heat exchange. U.S. Pat. No. 6,269,644 discloses a more recent absorption power cycle. U.S. Pat. No. 5,692,393 discloses a countercurrent mass exchange shell side desorption with countercurrent heat exchange by a single helical coil. U.S. Pat. No. 5,729,999 discloses a countercurrent mass exchange absorption using helical rods inside multiple cylinders. U.S. Pat. No. 5,557,946 and Swiss Patent 272,868 disclose additional cylindrical coil in shell sorbers. A variety of shell and coil heat exchangers are commercially available for liquidxe2x80x94liquid heat exchange or condensing heat exchange. Absorption power cycles and dual function absorption cycles are disclosed in U.S. Pat. No. 6,269,644.
Sorption is frequently accompanied by a substantial temperature glide, which can be beneficial to the overall transfer process, provided the heat transfer is counter-current, and provided there is no global recirculation of the liquid sorbentxe2x80x94local recirculation is beneficial, per U.S. Pat. No. 5,766,519. The volumetric flow rate of vapor may change during the sorption process by an order of magnitude or more. Similarly, the required flow rate and volume of heat transfer fluid can vary widely, and the large temperature glide may require a large number of transfer units. When using aqueous ammonia as working fluid, all-welded construction is desirable. Nickel-based brazing is acceptable for some metal-joining operations, but it is costly. The sheet metal configurations typically require such brazing, or substantial amounts of precision welding. Conventional shell and straight or U tube geometries must have relatively large spacing between tubes, too large for the desired tortuous flow path, owing to minimum tube-to-tube clearances at the tube sheet. For co-current upflow geometries, the hydrostatic pressure head may become excessive at heights above about 2 m, which restricts tube length in shell and tube configurations with counter-current heat exchange.
Thus, what is needed, and included among the objects of this invention, is a heat and mass transfer device for diabatic sorption with a volatile sorbent, which:
achieves a tortuous and/or turbulent flow path across the heat transfer surface by the sorbate vapor and sorbent liquid, such that the vapor-liquid interface is continuously renewed;
has counter-current heat exchange with a heat transfer fluid;
establishes and maintains good distribution of both fluid phases;
is adaptable to either co-current or counter-current mass exchange;
is preferably highly compact with all welded joints;
accommodates major variations in vapor and/or liquid loading; and
preferably can have multiple separate heat transfer fluids, in parallel and/or series.
The above and additional useful objects are achieved by providing a sorber comprised of:
a) at least three concentric coils of tubing contained in a shell;
b) a flow path for liquid sorbent in one direction through said sorber, into a sorbent entrance port and out of a sorbent exit port;
c) a flow path for heat transfer fluid through said sorber which is in counter-current heat exchange relationship with said sorbent flow path;
d) a sorbate vapor port which is in communication with at least one of said sorbent ports;
e) wherein each coil is coiled in opposite direction to those coils adjoining it, whereby an opposed slant tube configuration is obtained; and
f) wherein there is structure for flow modification in the core space inside the innermost coil.
The close juxtaposition of tubes slanted in one direction in a particular coil and in the opposite direction in the adjoining coil(s) gives rise to a tortuous, sinuous flow path which is known to provide an excellent liquid phase heat transfer coefficient. For sorption, it has been discovered to additionally overcome the traditional high resistance to heat and mass transfer encountered in volatile sorbent sorption processes provided the coil-to-coil spacing is less than about 4 mm and preferably less than 1.5 mm. It is theorized that this is the result of the tortuous flow path acting on the vapor-liquid interface to continuously distort it, and hence counteract the concentration gradients which otherwise build up at the interface. This action is further abetted by pressure equalization flow paths between the inside and outside of each coil, which tend to keep the sorbent and sorbate well distributed across the coils, especially in co-current upflow sorption. Surprisingly, there is substantial fluid flow area available even when the coil-to-coil spacing is zero. This is only true with the disclosed opposed slant tube configuration.